package/dvb-apps: requires iconv

Use libiconv if the toolchain does not have locales.

Fixes (for example):
    http://autobuild.buildroot.net/results/c5776c95ac128f7c7eadc2fe746ad89f8c1ef49a



[Peter: fixup LDFLAGS for BR2_ENABLE_LOCALE=y builds]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>